To confirm or deny the thesis that mitral anular dilatation is a cause of mitral regurgitation, this anulus was measured in 102 patients, 24 of whom had normal hearts and 78 of whom had dilated left ventricles, some associated with mitral regurgitation, others not. Dilatation of the mitral anulus is a rare cause of mitral regurgitation and that abnormality of the fibrous skeleton of the heart is necessary for the mitral anulus to dilate enough to cause mitral regurgitation.